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  <div class="question_difficulty">
   难度：Easy
  </div>
  <div>
   <h1 class="question_title">
    101. Symmetric Tree
   </h1>
   <p>
    Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
   </p>
   <p>
    For example, this binary tree
    <code>
     [1,2,2,3,4,4,3]
    </code>
    is symmetric:
   </p>
   <pre>
    1
   / \
  2   2
 / \ / \
3  4 4  3
</pre>
   <p>
    But the following
    <code>
     [1,2,2,null,3,null,3]
    </code>
    is not:
    <br>
   </p>
   <pre>
    1
   / \
  2   2
   \   \
   3    3
</pre>
   <p>
    <b>
     Note:
    </b>
    <br>
    Bonus points if you could solve it both recursively and iteratively.
   </p>
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   <h1 class="question_title">
    101. 对称二叉树
   </h1>
   <p>
    给定一个二叉树，检查它是否是镜像对称的。
   </p>
   <p>
    例如，二叉树&nbsp;
    <code>
     [1,2,2,3,4,4,3]
    </code>
    是对称的。
   </p>
   <pre>    1
   / \
  2   2
 / \ / \
3  4 4  3
</pre>
   <p>
    但是下面这个&nbsp;
    <code>
     [1,2,2,null,3,null,3]
    </code>
    则不是镜像对称的:
   </p>
   <pre>    1
   / \
  2   2
   \   \
   3    3
</pre>
   <p>
    <strong>
     说明:
    </strong>
   </p>
   <p>
    如果你可以运用递归和迭代两种方法解决这个问题，会很加分。
   </p>
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